The paradigm shift from consuming non-renewable fossil fuels to utilizing renewable biofuels necessitates economic supply chains and cheaper manufacturing routes in order for biofuels to have a competitive edge in the market. Biodiesel, a renewable fuel source, is both biodegradable and has a lower emission profile when compared to fossil fuels, rendering biodiesel as environmentally beneficial. Biodiesel is becoming more popular in both developed and underdeveloped countries since this fuel source can be synthesized from feedstocks such as animal fats, waste cooking oil, vegetable oils, amongst others renewable sources. Still, the production cost of biodiesel greatly relies on the selection of oil feedstock. Many cheap feedstocks for biodiesel production contain increased concentrations of free fatty acids (FFAs) such as waste fats and non-edible oils. Some of the most common FFAs include oleic, linoleic, linolinic, and palmitic acids. FFAs undergo saponification in the presence of a strong base used in the transesterification of triglycerides for the production of biodiesel, which compromises the overall biodiesel yield from different feedstocks and causes complications with the separation of biodiesel from alcohol. To prevent this problem, FFAs can be converted to usable biodiesel according to Equation (1) where a catalyst mediates the esterification reaction between the FFA and methanol:

Since inexpensive feedstocks, such as waste cooking oil, contain high concentrations of FFAs, there exists a need for inexpensive catalysts that can better convert FFAs to usable biodiesel.
Homogeneous catalysts, such as H2SO4, have been widely demonstrated to catalyze both esterification and transesterification reactions for the production of biodiesel. However, H2SO4 suffers from several drawbacks, namely the fact that H2SO4 causes equipment corrosion and the resulting biodiesel necessitates neutralization. To avert this problem, different heterogeneous solid acid catalysts have been explored since such catalysts can be easily removed by filtration and reused for subsequent reactions. The utilization of heterogeneous catalysts also allows for greater yields of biodiesel while averting the unwanted saponification reaction from FFAs. Different materials have been demonstrated as heterogeneous solid acid catalysts, including Nafion, zeolites, La/zeolite beta, Amberlyst-15, among others. Although such catalysts show good activity to catalyze the esterification of FFAs, these materials come at an increased cost, which comprises the overall biodiesel production price. For this reason, the exploration of cheaper heterogeneous catalyst systems is necessitated to allow biodiesel to have a competitive edge in the market.
Recently, there has been increased interest in pyrolyzed waste tires since they serve as a cheap carbon source that can be used in various applications including Li-ion battery anodes, supercapacitors, and catalysts. Approximately 290 million tires are disposed annually in the United States, and it is estimated that 27 million scrap tires end up in landfills or monofills annually. Since tire rubber is non-biodegradable, this material poses inherent environmental concerns and serves as one of the most problematic sources of waste. The pyrolysis of different waste carbon materials to produce carbon black has been demonstrated. For example, the pyrolysis of wood, sucrose, and coconut shells produces carbon black, which costs $0.80/kg, $0.25/kg, and $0.25/kg respectively. In comparison, the pyrolysis of waste tire carbon costs about $0.06/kg, rendering this material as an attractive candidate to produce activated carbon. In turn, waste tires can be used for multiple applications rather than their disposal into landfills.
Carbon-based solid acid catalysts have been widely explored where their use in catalysis is mainly focused on the esterification of FFAs. These catalysts have been prepared from different sulfonated carbon sources, such as sugars, polymers, and high surface area silica-templated carbons. These materials contain sulfonic acid groups (—SO3H), which catalyze the esterification of FFAs. The soft carbons show good initial performance in converting FFAs to monoesters, however, with increased use these carbons tend to leach acid sites, compromising their use to mass produce biodiesel. Therefore, more robust carbon supports are desired for biodiesel production. In addition to converting FFAs to monoesters (biofuel), the triglycerides present in used oil can be converted to esters (biofuel) and glycerin using the standard base catalysts.